The instant invention relates to a blending of hydrocarbon resin into epoxy resin and subsequently curing the epoxy to form a modified thermoset alloy of hydrocarbon resins in a cured epoxy matrix. Hydrocarbon resins of utility in the instant invention are formed by an alkylation reaction between cyclic diolefins such as dicyclopentadiene (DCPD) and aromatic solvents.
The alloys of the instant invention are of particular utility as binders for use with matrix materials, such as glass fabrics, for use in making laminate products for electronics.
Epoxy resins are commonly used as binders for making glass fabric reinforced laminate products for electronics applications. In these applications, epoxy resin, combined with a curative, is applied in liquid form (neat or as a solution) to glass fabric. The fabric containing the epoxy is then dried at elevated temperatures to remove volatiles from the epoxy and to partially cure the epoxy thereby yielding an intermediate product typically termed a xe2x80x9cprepregxe2x80x9d. Laminates can be constructed from prepregs by layering sheets of prepreg in various combinations and orientations and typically placing copper foil layer on one or both of the outside layers of the laminate to form a laminate structure. The laminate structure is then compressed with the application of heat and pressure to form electronic laminate products comprised of metal foil(s) bonded to cured epoxy/glass fabric sheets.
Typically, epoxies used in electronic laminate are Bisphenol A diglycidylether epoxy resins where a required amount of bromine has been reacted onto an epoxy molecule to confer flame retardance to cured product. Laminate produced from this type of epoxy is typically classified as FR-4 type.
Cured epoxy resins provide several desirable properties required for electronic laminate application such as infusibility, high glass transition temperature, solvent resistance, adhesion to copper, flame retardance, among other characteristics. However epoxy resins are inherently polar in character, and this polarity causes other undesirable properties commonly associated with epoxy resins such as high moisture absorption and higher dielectric constant and dissipation factor (higher than measured for many common polymers).
One potential technique for producing a material which retains the desirable attributes of epoxy resin and desirable attributes associated with a non-polar material is to form an alloy of non-polar polymer(s) blended into and cured with the epoxy resin itself. An example of such an alloy is a product produced and sold by General Electric as Getek(copyright) polymer. The alloy comprises an alloy of polyphenyleneether (PPE) polymer mixed with an epoxy resin, where this alloy is applied and used as the binder in electronic laminate products. While this alloy offers some property advantages, it suffers from the fact that the PPE polymer and epoxy resin are highly incompatible. This incompatibility leads to both processing problems and product variability. The latter problem is attributed to difficulties associated with consistent formation of a desired phase separation between the PPE and epoxy.
One solution to this problem is to use a non-polar hydrocarbon polymer which is soluble and compatible with epoxy resins. However non-polar polymers such as polypropylene, polyethylene, polystyrene, PPE, or hydrocarbon rubbers such as polyisoprene, polybutadiene, and the like are incompatible with epoxy resins. Compatibility can be improved by reducing the molecular weight of the polymer. One type of non-polar low molecular weight polymer are materials typically classified as hydrocarbon resins. Examples of such hydrocarbon resins include Kristalex(copyright) 3100 resin or Picco(copyright) 5140 resin available from Hercules Incorporated. These materials are low molecular weight polymers derived from pure styrenic monomers in the former case or mixed unsaturated aromatic feedstocks in the latter case. However even low MW oligomers such as these hydrocarbon resins display poor solubility and compatibility with epoxy resins.
The instant invention relates to an epoxy/alkylation-type hydrocarbon resin alloy comprising an epoxy resin, an alkylation-type hydrocarbon resin, and a curative. The epoxy resin may be a Bisphenol A diglycidylether epoxy resins where a required amount of bromine has been reacted onto an epoxy molecule. The alkylation-type hydrocarbon resin is characterized as a product formed by an alkylation reaction between a cyclic diolefin and aromatic solvent. One particularly effective alkylation-type hydrocarbon resin in this invention is the alkylation resin formed by the reaction of dicyclopentadiene (DCPD) with alkylnaphthalene solvent. The resin exhibits a high glass transition temperature (Tg) at very low number average molecular weight values (MW), less than about 10,000, preferably less than 5,000, most preferably less than 3,000, measured by size exclusion chromatography. The alkylation-type hydrocarbon resin is added to the epoxy resin at levels up to about 25% by weight, preferably between about 10% to 25% by weight, more preferably about 17.5% to 20% by weight.
The instant invention also relates to a process of producing an epoxy/alkylation-type hydrocarbon resin alloy comprising the steps of: dissolving an alkylation-type hydrocarbon resin in a liquid epoxy to form a mixture; adding a curative to the alkylation-type hydrocarbon resin/epoxy mixture; impregnating a matrix with the alkylation-type hydrocarbon resin/epoxy mixture containing the curative; and advancing the epoxy""s cure to provide the epoxy/alkylation-type hydrocarbon resin alloy.
Additionally, the invention relates to a prepreg comprising; a matrix impregnated with the epoxy/alkylation-type hydrocarbon resin alloy as well as laminates produced from such prepregs.
Alkylation hydrocarbon resins is an alternate way to hydrophobically modify the properties of epoxy materials. use of an alkylation-type hydrocarbon resin reduces processing difficulties associated with PPE alloys and forms a product which is hydrophobically modified.
One particularly effective alkylation-type hydrocarbon resin for this application is the alkylation resin formed by the reaction of dicyclopentadiene (DCPD) with alkylnaphthalene solvent, described in U.S. Pat. No. 5,391,670. Because of the rigid molecular structure of this material, it exhibits a high glass transition temperature (Tg) at very low number average molecular weight values (MW), less than about 10,000, preferably less than 5,000, most preferably less than 3,000, measured by size exclusion chromatography. Because of its aromatic character and low MW, the alkylation-type hydrocarbon resin can be readily dissolved into various epoxy resins and easily processed into resin impregnated glass cloth prepreg for ultimate conversion into electronic laminate products. These epoxy/alkylation-type hydrocarbon resin alloys retain high Tg values with minimal increase in thermal expansion. Incorporation of the non-polar alkylation-type hydrocarbon resin into an epoxy reduces moisture sensitivity and improves dielectric properties of the epoxy relative to the unmodified epoxy. Because of compatibility between in the epoxy and alkylation resin, other important properties such as degree of cure, solvent resistance, mechanical properties, thermal stability, and adhesive characteristics are not deteriorated despite the fact that the alkylation-type hydrocarbon resin does not co-cure with the epoxy matrix and retains its thermoplastic characteristics in epoxy/alkylation-type hydrocarbon resin alloy.
Alkylation-type hydrocarbon resin formed by the alkylation reaction between cyclic diolefins such as dicyclopentadiene (DCPD) and aromatic solvents exhibit almost universal solubility in a range of solvents and are highly soluble and compatible in various types of epoxy resins. Because of this high degree of compatibility, up to 25 vol % of alkylation-type hydrocarbon resin can be blended into epoxy resins and the mixture subsequently cured to form a modified thermoset alloy of alkylation-type hydrocarbon resin in a cured epoxy matrix. Due to the alkylation-type hydrocarbon resin""s hydrophobic character, these alloys exhibit reduced moisture sensitivity and improved dielectric properties. One application for alloys of this type is in electrical laminate products where improved dielectric properties and lower moisture sensitivity are highly desirable properties for the binders used in these products. Because of the compatibility between these alkylation resins and epoxy materials, the alloys retain good mechanical properties, solvent resistance, adhesion properties, and thermal stability, within the limits of compatibility of the alkylation-type hydrocarbon resin in epoxy matrix.
One particular class of oligomers found to be compatible in epoxy resins despite its non-polar character. This class is characterized as the product formed by an alkylation reaction between a cyclic diolefin and aromatic solvent. An example of such a material is the hydrocarbon resin described in U.S. Pat. No. 5,391,670, incorporated herein by reference in its entirety, which was found to be particularly suited for use as a non-polar, compatible polymer for alloying with epoxy resins. The resin(s) described in this patent are products derived from an alkylation reaction between diolefins and polycyclic aromatic species. Preferably the resin, being the alkylation products formed by reaction between dicyclopentadiene (DCPD) and a mixed alkylnaphthalene solvent. Because of this product""s rigid structure formed by combining cyclic naphthalene structures to the olefin sites of a rigid DCPD structure, the resultant resin exhibits a high glass transition temperature (Tg) at very low molecular weight values. Such a material can exhibit a Tg of around 80xc2x0 C. at a xe2x80x9ctruexe2x80x9d number average MW of about 400 Daltons. Also, because of the character of alkylation reaction, the resin formed through reaction of DCPD contains a high percentage of aromatic solvent residues in its structure. Thus, although the resin is non-polar and hydrocarbon in nature, it is highly aromatic and is more compatible with polar materials than aliphatic-type materials.
Other alkylation-type hydrocarbon resins suitable as compatible modifiers for epoxy resins would be an alkylation product formed by an reaction between DCPD and a benzene aromatic solvent, such as xylene. An example of this type of material is Piccovar(copyright) L-60 resin available from Hercules Incorporated. However due to the smaller size of benzene""s aromatic structure, alkylation-type hydrocarbon resins formed from benzene exhibit lower molecular weights and glass transition temperatures than alkylation-type hydrocarbon resins derived from alkylation of alkyl naphthalene solvents. Typical resins derived from alkylation of benzene exhibit a Tg about 50xc2x0 C. to 70xc2x0 C. lower than similar products made by alkylating naphthalene solvents, such as the products described in U.S. Pat. No. 5,391,670.
In practice a process for producing an epoxy/alkylation-type hydrocarbon resin alloy comprises the following steps. In a first step, the alkylation-type hydrocarbon resin is dissolved in liquid epoxy resins or solutions of epoxy resin in solvents such as MEK or acetone. Resins described in U.S. Pat. No. 5,391,670 were found to be soluble in common epoxy resins such as Epon 828 liquid epoxy resin available from Shell Chemical, or Epon 1123-A80, an 80% solution of a brominated Bis A epoxy resin in acetone, also available from by Shell Chemical. In a second step, a curative is added to the alkylation-type hydrocarbon resin/epoxy mixture. A variety of materials well known in the art may be used to cure the epoxy contained in the alkylation-type hydrocarbon resin/epoxy mixture. The curing agent may be selected from the group consisting of dicyanodiamide (DICY) and imidazole. Typically, the curative is added as a solution in a solvent, and typically the solvent is a polar solvent.
In electrical laminate production, typical curative solutions consist of dicyandiamide and a hindered amine co-catalyst dissolved in a polar solvent such as dimethylformamide (DMF) or a mixture of DMF and alcohols or other polar solvents. In the third step, a matrix, preferably glass cloth, is impregnated with the alkylation-type hydrocarbon resin/epoxy mixture containing the curative and any associated solvents and passed through an oven to remove volatile solvent and advance the degree of cure of the epoxy to provide a resultant epoxy/alkylation-type hydrocarbon resin alloy exhibiting a requisite material flow when subsequently laminated.
The alkylation-type hydrocarbon resin of this invention remains dissolved in the epoxy as a binder when it is applied to the matrix and does not substantially hinder coating, drying, or laminating of prepreg containing this resin as a binder. The alkylation-type hydrocarbon resin can be incorporated into the epoxy at levels up to around about 20%-25% by weight before saturation or compatibility limit of alkylation-type hydrocarbon resin in the epoxy is reached. Above the compatibility limit, properties of the epoxy/alkylation-type hydrocarbon resin alloy begin to deteriorate. Below the compatibility limit, important properties such as infusibility, solvent resistance, mechanical strength, adhesive properties, thermal stability, and thermal expansion are little affected.
In terms of volume fraction, up to around 25 vol % of the alkylation-type hydrocarbon resin can be incorporated into the epoxy before the compatibility limit is reached. Alloying the alkylation-type hydrocarbon resin into the epoxy reduces moisture sensitivity of the epoxy. Dielectric properties are also modified, the alkylation-type hydrocarbon resin causing a reduction in dielectric constant and dissipation factor. Adding the alkylation-type hydrocarbon resin does cause a modest reduction in the Tg of the cured epoxy/alkylation-type hydrocarbon resin alloy relative to the Tg for the epoxy itself. These materials cause only a modest Tg reduction of 5xc2x0-15xc2x0 C. even at high loading of 20-25 vol %. While alkylation-type hydrocarbon resin addition has a moderate effect on Tg, its effect on thermal expansion is even less pronounced. Thermal expansion of the epoxy/alkylation-type hydrocarbon resin alloy, measured by heating samples of the material from 25xc2x0 C. to 250xc2x0 C. in a thermo-mechanical analysis (TMA) apparatus, is little affected by incorporation of the alkylation-type hydrocarbon resin. Although adding alkylation-type hydrocarbon resin causes a modest reduction in Tg, cured binder compositions exhibiting Tg values in the range 120xc2x0 C. to 140xc2x0 C. can be prepared from these alkylation-type hydrocarbon resin alloyed with conventional brominated Bis A epoxy resin.